Herebelow, numerals in square brackets—[ ]—are keyed to the list of references found towards the close of the present disclosure.
In the general realm of direct-conversion radio receivers operating at high frequencies, it is well known that leakage of the local-oscillator (LO) signal into the RF-signal path can generate cross-modulation and inter-modulation distortion and time-varying DC offsets in the mixer stage of the receiver. Since the LO signal and the RF signal are at the same frequency, the LO signal which leaks into the RF-signal path is indistinguishable from the RF signal itself, and the LO leakage is mixed with the LO signal itself in the mixer to appear as a distortion or noise component in the mixer's baseband output.
Accordingly, it has been found to be desirable to minimize the leakage of the LO signal into the RF-signal path. Conventionally, this has been done at least in part by laying out the circuits so as to minimize the coupling between the LO and RF signal paths. In the mixer itself, double-balanced mixers are preferred [1,2] since they provide generally good isolation between the LO port and the RF-input port.
However, at sufficiently high frequencies, LO-to-RF isolation of even a double-balanced mixer can become inadequate. Inherent imbalances in the circuit due to imperfect component matching results in incomplete cancellation of the LO fundamental at the common emitters of the switching quad, and leakage of the LO fundamental frequency to the RF input occurs through the base-collector junction capacitance of the RF-input transistors.
Accordingly, a need has been recognized in connection with improving upon the shortcomings and disadvantages experienced with conventional arrangements as discussed above.